In vehicle accessory drives for use with an automotive internal combustion engine, it is desirable to establish a multiple ratio driving connection between an engine driven element and an engine driven accessory such as an alternator. Thus, it is possible to drive the alternator with a reduced speed ratio when the engine speed is high and at a higher speed ratio when the engine speed is low.
Internal combustion engines are characterized by a wide range of engine speeds throughout the normal driving cycle. The engine driven accessory, such as the alternator, thus must be designed for the required capacity when the vehicle engine speed is low. If a fixed ratio drive exists between the engine driven accessory and the engine, the accessory speeds will be excessive for the design capacity of the accessory when the engine speed increases. This tends to create an undesirable parasitic power loss.
I am aware of various two-speed accessory drives that are intended to increase the speed ratio at low engine speeds relative to the speed ratio that exists at high engine speeds. These include friction belt drives such as the crankshaft pulley and clutch system of U.S. Pat. No. 4,080,843, which discloses an accessory that is driven directly by the crankshaft from idle speed to a predetermined speed level. A suitable clutch arrangement is used to establish a reduced accessory speed relative to the crankshaft speed during operation of the engine at high speeds. Slipping clutch members are used between the crankshaft and the accessory drive pulley to provide a time interval of substantially constant accessory rpm during which the crankshaft is released from the torque input shaft for the accessory as the camshaft becomes drivably connected to the accessory through a one-way clutch. The camshaft, of course, is driven at one-half the engine crankshaft speed.
I am aware also of geared two-speed accessory drives for use in establishing a geared torque flow path between an engine driven pulley and the accessories. Examples are shown in U.S. Pat. No. 4,854,192 and U.S. Pat. 4,862,770. In the case of prior art '770 patent, an engine driven pulley is connected to an accessory drive shaft through a planetary arrangement comprising two epicyclic differential diameter gears and a pair of sun gears of unequal pitch diameter. The torque reaction for the two-speed accessory drive is obtained by a brake band encircling a brake drum connected to the carrier. Such two-speed accessory drives generally exhibit undesirable gear noises and lack economy of space within the engine compartment of an automotive vehicle. Furthermore, they are difficult to manufacture, and often are costly and thus are not commercially feasible for large volume automotive vehicle uses.